It is well documented that when parallel low-side compressors are employed in closed refrigeration systems the tendency exists for one of the compressors to become starved for lubricating oil. A lowside compressor is one in which suction gas is essentially dumped into the interior of the shell of the compressor.
The closed shell of a low-side refrigeration compressor houses a motor-compressor unit and generally defines a lubricating oil sump at its bottom. A portion of the motor-compressor lubricating oil, which collects and is stored in the sump area, becomes entrained in the suction gas which dumps into the shell of the compressor and travels with the suction gas into, through and out of the compressor. The entrained oil flows with the refrigerant into the remainder of the refrigeration system and is carried back into the shell of the compressor with the suction gas as it returns from the evaporator.
When suction gas is returned from the evaporator to the compressors in a refrigeration system having parallel low-side compressors it is inevitable that one of the compressors will draw more suction gas, and consequently more entrained lubricating oil, into its shell than will the other compressor. Over a period of time and unless otherwise accounted for, the oil in the sump of one of the compressors will be depleted while the shell of the other compressor will become overfilled with oil. Provision must therefore be made to equalize the oil levels in the parallel compressors of such refrigeration systems and to maintain those levels in an equalized state during system operation. Failure to do so can result in the catastrophic failure of the compressor whose oil supply becomes depleted.
Many attempts have been made to solve the oil equalization problems associated with compressors in parallel compressor refrigeration systems. Many such attempts have been based upon the mechanical pumping of oil from one compressor sump to the other. Other solutions to the oil equalization problem focus upon equalizing the pressures in the sumps of parallel compressors to insure that equal amounts of suction gas, and therefore entrained lubricating oil, are continuously and independently delivered to the shell of each compressor. Both of these solutions are relatively complex and are generally subject to mechanical breakdown.
Because of the relative complexity of such systems and the catastrophic results which can occur upon their failure, efforts have been made to provide oil equalization arrangements in parallel compressor refrigeration systems which are more mechanically simple and therefore more inherently reliable than the previously mentioned oil level equalization schemes. Exemplary in this regard are U.S. Pat. Nos. 3,386,262 to Hackbart and 3,785,169 to Gylland, the former being assigned to the assignee of the present invention.
In Gylland a parallel compressor lubrication scheme is taught which is based upon the delivery of the entire volume of suction gas from the evaporator in a refrigeration system to a single one of the two parallel discharge compressors therein. Suction gas is then communicated from the shell of the first compressor to the shell of the second compressor. Gylland teaches, therefore, a series input, parallel output arrangement. Because of this arrangement, the shell of the compressor to which suction gas is directly delivered is always at a higher pressure, when the system is in operation, than the shell of the downstream compressor. The higher pressure in the first compressor is employed to drive oil from the sump of that compressor to the sump of the second compressor. Most significant in the Gylland arrangement is the avoidance of parallel suction paths into the shells of parallel output compressors.
In Hackbart, refrigerant is directed from the evaporator in a refrigeration system to a "T" or "Y" shaped coupling which has a branch line connection. Because of the coupling configuration, the shell of a first of the compressors receives a majority of the suction gas and therefore, a majority of the lubricant entrained therein. By virtue of the delivery of a majority of the suction gas to it, the shell of the first compressor is maintained at a higher pressure than that which will be found in the shell of the second compressor when the first compressor is in operation. The second compressor relies upon the receipt of oil from the shell of the first compressor through an oil equalization conduit. Oil is driven from the shell of the first compressor through the equalization conduit by the elevated pressure in the shell of the first compressor. However, the coupling in Hackbart is configured so as to also allow for the delivery, through a conduit connected to the branch line connection of the coupling, of refrigerant gas and some lubricating oil directly to the shell of the second com- pressor.
In the Hackbart coupling the branch line connection which leads to the shell of the second compressor is completely out of line with the flow path of suction gas and oil which enters the coupling from the evaporator. The line leading to the first compressor from the coupling is directly in line with the suction gas flow path. No provision exists by which suction gas and/or oil is positively acted upon and diverted into the branch line which leads from the coupling to the second compressor. Thus, there is no facility in the Hackbart coupling which positively acts upon the suction gas flow stream to insure the direct delivery of at least a portion of the oil entrained in the suction gas to the sump of the second compressor. Further, because of the inertia of the suction gas flow stream and the radical direction change required of it to enter into the branch line leading to the second compressor, the Hackbart coupling tends to promote the disentrainment of the heavier oil from that portion of the suction gas which is able to accomplish the extreme change in direction of travel which is required before it can enter the branch line.
It has been determined that somewhat more active rather than passive oil management is preferable in parallel compressor refrigeration systems than is accomplished by the Hackbart coupling. Yet it has long been recognized that the reliance upon mechanically operated apparatus such as pumps to accomplish active oil management can unnecessarily complicate a refrigeration system and lead to the catastrophic failure of a compressor therein should a mechanical malfunction occur. Therefore, the need continues to exist for apparatus which positively provides for and encourages the direct delivery of suction gas and entrained oil to both of the compressors in a parallel compressor refrigeration system yet which maintains the mechanical simplicity of a system not subject to a malfunction of a mechanical nature.